1. Field of the Invention
The present invention relates to a transparent toner which is used when a color image is formed by an electrophotographic system, an electrostatic recording system, or the like, and a method and apparatus for forming a color image. More particularly, the present invention relates to a transparent toner, a color image forming method, and a color image forming apparatus, in which graininess, color reproducibility, smoothness, and fixing strength are sufficiently fulfilled synchronously without being deteriorated and an arbitrary glossiness can be uniformly reproduced on a transfer material without depending on image density.
2. Description of the Related Art
Conventionally, a color image is formed on a transfer material (i.e., a color copy is obtained) by an electrophotographic system or the like in such a manner as described below. First, light is applied onto an original. The light reflected by the original is separated into multiple colors by a color CCD, is subjected to image processing by an image processing apparatus, and is further subjected to color correction so as to obtain image signals of multiple colors. The image signals are made into laser light modulated by using, for example, a semiconductor laser for each color. The laser light is irradiated on an inorganic photosensitive material such as selenium (Se) and amorphous silicon, or on an organic photosensitive material in which a phthalocyanine pigment, bis-azo pigment, or the like is used for a charge generating layer, a plurality of times (i.e., one color at a time) so as to allow formation of a plurality of electrostatic latent images. The plurality of electrostatic latent images are developed sequentially by using, for example, four color toners of Y (yellow), M (magenta), C (cyan), and K (black). The developed color toner images are transferred from the inorganic or organic photosensitive material onto a transfer material such as paper and are heated and fixed thereon by a heat-fixing roll or the like. As a result, a color image is formed on the transfer material.
Meanwhile, in the above-described case, the color toner is, for example, prepared with a colorant being dispersed in a binding resin such as polyester resin, styrene/acrylic copolymer, and styrene/butadiene copolymer and an average particle diameter of the color toner is approximately 1 to 15 .mu.m. Fine particles each having the average particle diameter of approximately 5 to 100 nm, for example, inorganic fine particles such as silicon oxide, titanium oxide, and aluminium oxide, or fine resin particles such as PMMA and PVDF adhere to the surface of the color toner. Further, examples of the yellow colorant include bendizine yellow, quinoline yellow, and hanza yellow and examples of the magenta colorant include rhodamine B, rose bengal, and pigment red. Examples of the cyan colorant include phthalocyanine blue, aniline blue, and pigment blue and examples of the black colorant include carbon black, aniline black, and a blend of color pigments.
The surface of the color image formed as described above is made smooth during heat-fixing processing, and therefore, the color image has a glossiness which is different from that of the surface of a paper. Further, it has been known that the viscosity of the color toner during the heat-fixing processing varies depending on the kind of the binding resin contained in the color toner and the type of heat-fixing system, and the glossiness of the obtained color image thereby varies.
On the other hand, the preferable degree of glossiness of the color image widely varies depending on the types of images, the purposes for use, and the like. In the case of a photographic original of a human figure or landscape, a high-glossy image is generally preferable from the viewpoint of obtaining a sharp image. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 5-142963, 3-2765, and 63-259575 each disclose that a high-glossy image is obtained by using a color copying machine and by selecting the materials of the toner and the fixing conditions. However, in this case, although the glossiness of an image portion obtained by the toner can be made higher, the glossiness of a non-image portion cannot be made higher. As a result, the glossiness on the transfer material cannot be made uniform. Further, even a character image by which low glossiness is desired is brought into the state of high glossiness and the glossiness cannot be arbitrarily selected.
In order to solve the above-described drawbacks, Japanese Patent Application Laid-Open (JP-A) Nos. 4-204669, 1-142740, and 63-300254 each disclose that by changing the characteristics of the binding, resins in the color toner and in the black toner, a black image (black character) is provided to have a low glossiness and a color image is provided to have a high glossiness. However, in this case, although only the black image (black character) can be brought into low glossiness, the glossiness of a color image or a color character cannot be arbitrarily controlled. Further, the glossiness on the transfer material cannot be made uniform as is the above case.
In the case of a color image obtained by a color copying machine, as compared with an image obtained by printing or silver-salt photography, a low-glossy image has an irregularly roughened surface and irregular reflection occurs on the surface of the color image. For this reason, even if a high-dispersion pigment or a small-particle-diameter toner is used, graininess, glossiness, and color tone become deteriorated. Further, in the color image obtained by a color copying machine, the glossiness thereof varies widely depending on the density of the image, and therefore, glossiness, graininess, and color tone are each different between a background portion (a non-image portion) and a low-density portion and between the background portion and a high-density portion and a finished image gives a feeling of unnaturalness having no smoothness as compared with the image obtained by printing or silver-salt photography.
Accordingly, in order to solve these problems caused by the glossiness, it has been proposed that by changing a fixing speed, a fixing temperature, a fixing pressure, or a fixing system when a toner image is fixed onto a transfer material, the glossiness of a color image is arbitrarily changed. However, in the case of the above fixed image, the fixing strength in a low-glossy image cannot be sufficiently obtained, and when abraded or folded, the image is impaired extremely. Further, in the case of a high-glossy image, binding resin permeates into the transfer material and sufficient glossiness cannot be obtained accordingly.
In order to avoid the above-described phenomena, an increase in molecular weight of the binding resin in the toner, an increase of cross linking component, or broadening of molecular weight distribution is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 6-148935. However, in this case, although the fixing strength in the low-glossy image can be sufficiently realized, the high-glossy image cannot be sufficiently obtained. In other words, there exists a drawback in that it is difficult to synchronously fulfill the following objects: to obtain sufficient fixing strength in a fixed image; to obtain a high-glossy fixed image; and to prevent the binding resin from excessively permeating into the transfer material so that the state of the surface of the transfer material does not appear on the image surface. Further, in this case, each glossiness of a background portion (a non-image portion) and a low-density portion depends on the kind of the transfer material, and therefore, the glossiness on the transfer material cannot be made uniform as in the above case.
On the other hand, in order to solve the problem about the difference in glossiness caused by image density, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-58374, 4-278967, 4-204670, 7-72696, and 5-232840 each disclose a method in which in addition to color toners, a transparent toner is transferred onto and fixed on a transfer material. Further, Japanese Patent Application Laid-Open (JP-A) Nos. 63-92964 and 63-92965 each disclose a method in which transparent resin is previously applied onto a transfer material. However, in these methods, although a uniform glossiness can be obtained without depending upon image density, under the condition that a binding resin in a toner does not permeate into the transfer material and the fixing strength of a fixed image is sufficiently obtained, the glossiness of the color image is necessarily and univocally determined in accordance with the characteristics of the binding resin contained in the toner.
On the other hand, Japanese Patent Application Laid-Open (JP-A) No.4-204670 discloses that transparent toners containing a plurality of kinds of binding resins having different molecular weights are used and at least one of the transparent toners is transferred onto and fixed on a color image so as to form an image having a different glossiness thereon. In this case, developing machines are required by the number of various glossiness. However, the number of developing machines is actually limited and it is therefore difficult to obtain an arbitrary glossiness. Further, installation of a toner supply device or a developing machine requires an additional space, and therefore, the entire apparatus is made larger unnecessarily. Moreover, since a plurality of kinds of binding resins having different molecular weights is used, it is difficult to control an amount of charge and it is thereby impossible to obtain a stable amount of development. In addition, there also exist drawbacks in that binding resins having a large molecular weight for obtaining a low glossiness are generally inferior in productivity and the fixing temperature thereof becomes higher.
Accordingly, under the existing circumstances, there has not been provided a color image forming technique in which graininess, color reproducibility, smoothness, and fixing strength in a color image can be fulfilled synchronously and sufficiently and glossiness of the color image can be controlled arbitrarily without depending on image density.
Accordingly, the present invention intends to solve the above-described problems in the prior arts and to achieve the following object. In other words, it is the object of the present invention to provide a color image forming method and a color image forming apparatus, in which graininess, color reproducibility, smoothness, and fixing strength are fulfilled synchronously and sufficiently without being deteriorated and an arbitrary glossiness can be uniformly reproduced on a transfer material without depending on image density.